Its difficult to say what happened from the way you explain it. A colostomy is where an end of the colon is brought through the abdominal wall and sutured there. So when you say the colon became separated from the colostomy you must mean either the colostomy failed and the end of the colon sank back into the abdominal cavity or a part of the colon was anastomosed somewhere prior to the piece of colon used to make the colostomy and that the anastomosis came apart.

People who have taken longterm steroids for Crohn's disease have a thinning of their intestinal wall which makes it easy for staples or stitches to tear through it after surgery and you end up leaking intestinal contents into your gut.
Steroids and/or immune suppressants used for Crohn's disease also slow healing down drastically, so its possible that the stitches holding the colostomy to the abdomen started to dissolve before the intestine had healed to the abdominal skin enough to hold it there (though the stitches shouldn't have dissolved much in 8 days).

Unfortunately, infections can be deadly for people with Crohn's who have been suppressing their immune system for years. I'm sorry for your loss. I was in an identical situation with my fiance. Luckily he came through the 2nd surgery okay.

I would warn anyone considering surgery to get completely off prednisone for a while before your surgery.